This application relates to the field of digitized imaging. It is more specifically directed to image marking and determining distortion in and removing it from an image.
With the development of means of production and circulation of digital images, and the means of imbedding relatively invisible watermarks into digital images ostensibly to convey ownership of the image, there is now financial incentive to attempt to render an imbedded watermark undetectable. Pixel locations of a watermarked image are presumed to correspond to those in an unmarked original image. Generally, the watermark is imbedded by altering only the values of the pixel components of the original image, not their geometric positions. This may be accomplished employing such methods as described in U.S. Pat. No. 5,825,892 which is incorporated herein by reference in its entirety.
Some methods of attacking an imbedded watermark rely on constructing a new image, called a distorted image, that is based on the watermarked image. Pixels in the distorted image are placed at subtly distorted positions relative to those in the watermarked image. Pixel component values in the distorted image are determined by two-dimensional interpolation of component values of neighboring pixel in the watermarked image. To those skilled in the art it is obvious that excessive pixel position-distortion will cause the distorted image to become a caricature of the watermarked image, thus diminishing or destroying its economic value. Whether a distortion is excessive is a subjective measure. For a distorted image to be useful, it requires that whatever linear or nonlinear distortion methods that are used by an attacker have to be used sparingly, and in such a manner as to produce smoothly varying and relatively small position distortions. This is so as to be essentially unobjectionable and casually unnoticeable to untrained observers. The human visual system, as a qualitative measuring device, can be relied upon to readily detect excessive distortion.
It would be advantageous to have a method which distorts selective parts of an image but leaves other parts undistorted. The method would indeed form different types of caricature of the image. The method should be such that the distortion is reversible by the authority that inserted it. Furthermore, distortion upon distortion are selectively imported upon the image each level of distortion being reversible in its own right rather than just allowing removal of all distortion.
The present invention provides, firstly, methods and apparatus that reverse the notion of the attack process and use intentional image distortion as a means of indelibly marking an original image; it is, therefore, an alternative means of conveying image ownership. Secondly, intentional image distortion is used to further obscure an imbedded invisible watermark, such as one applied according to methods as described in U.S. Pat. No. 5,825,892 which is incorporated herein in entirety by reference.
In one aspect the method of the present invention utilizes an undistorted reference image relative to which measurements of distortion are made. For this purpose, either the unmarked original image or an undistorted but invisibly watermarked copy of the original image, called a watermarked image, can serve equally well as the reference image.